Physical Description

H. monstrosus is the largest bat found in Africa, with males being significantly larger than females. Males range in mass from 228 g to 450 g, averaging 377 g. Females are about half the size, weighing from 218 to 377 g, and averaging 275 g. Males range in length from 220 to 280 mm, with an average of 250 mm. Females measure 195 to 225 mm, averaging 210 mm. Males have a wingspan of 686 to 970 mm, and females have an average wingspan of 840 mm.

The species is sexually dimorphic in facial features as well as in size. Males have an enlarged rostrum and larynx, and a large, square head. They can also be distinguished by their huge, pendulous lips, flaps around a warty snout, a bald, split chin and cheek pouches. To human eyes, these bats are really ugly, prehaps accounting for their species name, "monstrosus." Females also have a square-shaped head, but their muzzle is fox-shaped and lacks the unique lip, snout, and chin characteristics of the male.

The ears are rounded at the base and the tail is very short. There is a claw on the second finger and this bat has large, flexible thumbs.

The pelage is slate-brown and there is a white collar of fur that stretches from shoulder to shoulder. The face and wing membranes are dark brown and the ears are dark brown with white tufts at the base.

Reproduction

H. monstrosus has a lek or arena mating system. The bats gather at night along a stream or river bed where males line up on tree branches in a long, narrow strip. They space themselves evenly in 10 m intervals and advertise themselves to females. An assemblage could contain from 25 to 132 bats.

Males attract females with loud vocalizations, including guttoral honking and croaking. They also use wing flapping. The bizzare head shape of the males of this species probably aids them in making favorable vocalization. Females fly through the arena to assess the males, then choose a mate and sit beside him on the branch.

Males emit a high buzz call when chosen and copulation begins. The duration of copulation is only 30-60 seconds and the pair leave independent of each other to forage. There are two mating sessions per night, one around 9:00 PM and the next at 4:00 AM. The early evening session is when most copulations occur and the later session is mainly for male agonistic behavior as they establish what territory in the strip will be theirs to display (Langevin and Barclay, 1990).

The species is highly polygynous. Nowak (1999) reports that only 6 percent of the males in a population accounted for 79 percent of matings. The successful males tended to be clumped together in the assemlage of males, indicating the importance of territory within the strip.

Data on when hammerhead bats breed vary. Some studies suggest that the peak of births occurs August to September during the middle of the rainy season and then another at the end of the rainy season, from October to December (Wolton et al., 1982). Others suggest the breeding peaks are at six month intervals and that are synchronized with the two dry seasons (Bradbury, 1977). In captivity, females maintain the same cycle they exhibit in the wild, breeding during June to August, and again in December to January (Nowak, 1999).

Females typically produce one offspring at a time, although twins have been seen. Newborn hammerhead bats weigh about 40 g (Nowak, 1999). Gestation length has not been reported for this species, nor has time until weaning. other Pteropodids vary greatly in these parameters, so it is difficult to speculate on this species based trends within the family.

The male H. monstrosus reaches sexual maturity at approximately 18 months and does not develop sexually dimorphic facial features until 12 months. Females are sexually mature at 6 months and full size at 12 months (Bradbury, 1977).

Parental care has not been reported for this species, but is likely to be the responsibility solely of females. Females provide milk, protection, grooming, and other essentials to the young in highly polygynous species. In some species, females carry their young with them while they fly around, and in others females park their young in roosts while they forage. There are no reports regarding what H. monstrosus does with its young.

Lifespan/Longevity

The hammerhead bat has a life expectancy of up to 30 years (Kulzer, 1990).

Average lifespanStatus: wild

30 years

Behavior

This species is nocturnal. The most studied behavior of hammerhead bats is the lek mating system (see Reproduction, above). H. monstrosus is social, roosting during the day in groups of up to 25, but with an average roosting group size of 4-5 individuals. Each individual spaces itself 10-15 cm from others, the only exception being a mother and her offspring. These bats roost in mixed age and sex assemblages, and they may change roosts daily (Langevin and Barclay ,1990).

Communication and Perception

Food Habits

Figs make up most of the H. monstrosus diet, but this bat may also include the juice and soft pulp of mangos, bananas and guavas. Van Deusan (1968) reports H. monstrosus showing some carnivorous behavior by attacking chickens to drink their blood and scavenging for meat.

Males and females use different foraging strategies. The females rely on established, dependable food resources of moderate quality. The males prefer to search out high quality food patches, flying up to 10 km to find the best food (Bradbury ,1981). This difference may reflect different metabolic needs, based on the difference in size between males and females.

Predation

To avoid predation, the hammerhead bat roosts high in forest canopy (20-30m), in groups of up to 25 individuals. During the day, they are inactive and rely on camouflage to hide them from predators (Kulzer, 1990).

Although humans and birds of prey may take some of these bats, parasites seem to be the main threat to their health. These have their own specific hepatoparasite, Hepatocystis carpenteri (Langevin and Barclay, 1990).

Ecosystem Roles

Although hammerhead bats are frugivorous, they consume mainly the juice and occasionally the pulp of their meal. Because of this, they are not very good seed distributors. However, given their size, they make a pretty good meal for a bird of prey and may affect prey populations, dependingupon their availability (Langevin and Barclay, 1900).

Economic Importance for Humans: Positive

There have been no reports of the affect of this bat on humans. However, humans do hunt these animals, as noted under Predation, above, so they do provide food.

Contributors

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

frugivore

an animal that mainly eats fruit

herbivore

An animal that eats mainly plants or parts of plants.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

nomadic

generally wanders from place to place, usually within a well-defined range.

polygynous

having more than one female as a mate at one time

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

territorial

defends an area within the home range, occupied by a single animals or group of animals of the same species and held through overt defense, display, or advertisement

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.